If you would like to change the OS of your vps, you need to take complete backup of any of your data on the vps and keep it at a safe place. Then you need to open a support ticket to re-create your vps with your desired OS. The OS change requires new vps setup so all previous data will be lost, so you must take backup which you will need to restore later on.
There is $15 charge for OS change. If you would like us to take the backup and then restore it for you, the manual work required to do that will cost $35/half hour.